1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing radially-oriented sintered ring magnets used in compact motors, for instance.
2. Description of the Background Art
A radially-oriented anisotropic ring magnet is often used in small-sized permanent magnet motors. The radially-oriented anisotropic ring magnet exhibits a generally rectangular magnetic field distribution pattern and, thus, one problem usually associated with a motor incorporating a radially-oriented anisotropic ring magnet is a high level of cogging torque.
A conventional approach commonly used for mitigating the cogging torque is to reduce distortion of the magnetic field distribution pattern of a ring magnet by skewed magnetization. This approach does not work so effectively, however, when it is necessary to suppress the cogging torque by a higher degree as in the case of a servomotor, for instance.
Another conventional approach to reducing the cogging torque is to form corrugations (hollows and protrusions) on a generally cylindrical outer surface of a ring magnet, the corrugations being skewed with respect to an axial direction of the ring magnet, as proposed in Japanese Patent Application Publication Nos. 1997-35933 and 2001-211581, for example. This approach makes it possible to reduce distortion of a magnetization distribution pattern in a rotating direction of the ring magnet by the corrugations as well as the cogging torque by virtue of the skewed corrugations.
Still another conventional approach to reducing the cogging torque caused by a radially-oriented anisotropic permanent ring magnet is shown in Japanese Patent Application Publication No. 1985-124812, the permanent ring magnet is manufactured by an injection molding method by using a metal die having hollows or protrusions formed at least in two areas on a curved inner surface or a curved outer surface or on both.
The ring magnets shown in the aforementioned Japanese Patent Application Publications are so-called bonded magnets which are produced by molding magnetic powder with thermosetting resin or thermoplastic resin used as a binder. Generally, magnetic force produced by the bonded magnets is so weak that the bonded magnets can not be used for manufacturing compact high-power motors. For example, a bonded rare-earth magnet produces a maximum energy product of about 10 to 25 MGOe which is low compared to an energy product of 40 MGOe produced by a typical sintered neodymium-ion-boron magnet. Since the magnetic force produced by the bonded magnets is so weak that the bonded magnets are not applicable to manufacturing servomotors which require a strong magnetic force.
The magnet shown in Publication No. 1997-35933 is a resin-molded magnet which must be formed by using a dedicated extruder. This extrusion molding process has a problem that the magnetic force of the resin-molded magnet which is weak by nature becomes still weaker because it is impossible to increase the magnetic force by applying a magnetic field during the molding process for anisotropically magnetizing the magnet.
Additionally, resin-molded magnets manufactured by the extruder are limited to shapes in which magnetic poles are obliquely formed, or skewed, with respect to an axial direction of the magnet. In a ring magnet used in a motor, however, magnetic properties of the magnet are not necessarily uniform along the axial direction, the ability of a magnetic circuit to conduct magnetic flux from the ring magnet to a stator, or permeance, varies along the axial direction, and saturation status of the stator varies along the axial direction. To cope with these problems, it is necessary to vary the shape of the magnet along the axial direction.
It would be possible to make a ring-shaped powder compact of which curved outer surface is corrugated with alternating hollows and protrusions which are skewed about a central axis of the ring-shaped powder compact by pressing magnetic powder by use of a conventional single-structured die. It is however impossible to draw out the ring-shaped powder compact from the conventional die. A compressive stress produced during a pressing process remains in the ring-shaped powder compact. Therefore, if one attempts to remove the ring-shaped powder compact from the die, a considerable friction force acts between the curved outer surface of the ring-shaped powder compact and a curved inner surface of the die, so that it would be necessary to pull the ring-shaped powder compact with a greater force than the friction force. In a case where skewed corrugations are formed on the curved inner surface of the die and on the curved outer surface of the ring-shaped powder compact, however, it is impossible to release the ring-shaped powder compact from the die by pulling the ring-shaped powder compact biased by the compressive stress while turning the same with a force greater than the friction force. The ring-shaped powder compact would break if forcibly drawn against the friction force due to the skewed corrugations.